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Powerpoint for Chapter 13 - PowerPoint PPT Presentation

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Alcohols contain an –OH group attached to a saturated (sp 3 ) carbon atom. Phenols contain an –OH group attached to one of the carbons of a benzene ring (sp 2 ). Ethers contain an –O- atom bonded to two carbon atoms which can be either aliphatic or aromatic.

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  • No isomers are possible for the simplest alcohols: methanol, CH3OH, and ethanol, CH3CH2OH. But two isomers are possible for C3H8O

  • For alcohols having the formula C4H10O, two different carbon skeletons are possible:

  • The –OH group can be placed in two unique positions on each carbon skeleton:

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  • Although not shown in the above illustration, each alcohol molecule is hydrogen bonded to several neighboring alcohol molecules.

  • The high melting and boiling temperatures of alcohols compared to the corresponding hydrocarbons is a direct result of the strengths of hydrogen bonds compared to London forces.

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  • The difference in boiling points between hydrocarbons and the corresponding alcohols decreases as the chain length of the hydrocarbon increases.

  • London forces are directly related to molecular size and the proportion of the total molecular force due to hydrogen bonding becomes less at higher molecular weights.

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  • Alcohols containing 4 carbons are moderately soluble, 5 carbons slightly soluble, and more than 5 carbons are negligibly soluble.

  • An alcohol containing 5 or more carbons is soluble only if it contains two or more alcoholic groups.

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  • An alcohol can accept a proton (act as a base) from strong acids such as sulfuric acid (H2SO4):

  • The equilibrium lies far to the left: only about 0.1% of the alcohol molecules become protonated, however, this small concentration is important in the dehydration reactions of alcohols.

  • The acidity of alcohols is too weak to be observed in reactions with strong bases. However they slowly react with active metals such as sodium to yield hydrogen gas:

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  • All alcohols can be dehydrated if they contain an H-atom on a carbon adjacent to the carbon holding the –OH group.

  • Methanol, for instance, cannot be dehydrated. It contains only one carbon.

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  • The dehydration of alcohols is the reverse of the hydration of alkenes.

  • LeChatelier’s principle allows the equilibrium position to be adjusted in favor of excess alcohol or excess alkene:

    • When hydration is desired, a large excess of water is used.

    • When dehydration is desired, the reaction is run at a temperature above the boiling point of the alkene formed, which distills out of the reaction mixture.

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  • In this case, 2-butene is the major product (the carbon with fewest H).

  • So, the more stable the alkene, the higher its yield in a dehydration reaction and the more substituted the alkene, the more stable it is:

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  • In the dehydration of 2-butanol previously shown, the predominate 2-butene product formed is a mixture of cis and trans isomers. The trans isomer predominates because it is more stable than the cis isomer.

  • Dehydration reactions occur during physiological processes such as glycolysis and the citric acid cycle.

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  • The weakest bonds in an alcohol are the O-H bond and the adjacent C-H bond. These bonds only are oxidized during selective oxidations.

  • Typical selective oxidizing agents are MnO4- and Cr2O7-.

  • Primary, secondary, and tertiary alcohols respond differently to selective oxidation:

    • Primary alcohols are oxidized in two stages:

      • Stage 1: Simultaneous loss of hydrogens (dehydrogenation) from the –OH group and the adjacent C-H carbon producing a carbonyl group and a resulting compound called an aldehyde.

      • Stage 2: Oxidation of the –H attached to the carbonyl group of the aldehyde to –OH, producing a carboxylic acid.

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  • Secondary alcohols: Oxidation cannot proceed beyond the carbonyl stage. These alcohols are oxidized to ketones.

    • Tertiary alcohols: Oxidation of the –OH cannot occur because no hydrogen atom is attached to the same carbon as the –OH group.

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    • Permanganate and dichromate can be used for simple chemical diagnostic tests for primary and secondary alcohols.

      • Permanganate oxidation: MnO4- (purple solution) is converted into MnO2 (brown precipitate)

      • Dichromate oxidation: Cr2O72- (orange solution) is converted into Cr3+ (green solution)

    • Uses and limitations:

      • Primary and secondary alcohols ARE distinguished from tertiary alcohols which are unaffected by permanganate and dichromate.

      • Primary and secondary alcohols ARE distinguished from alkanes, cycloalkanes, aromatics, and esters, which do not undergo permanganate or dichromate oxidations.

      • Positive permanganate or dichromate tests DO NOT distinguish primary and secondary alcohols from alkenes, alkynes, and phenols, which are also oxidized by these reagents.

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    • Phenols contain an –OH group bonded to one of the sp diagnostic tests for primary and secondary alcohols.2 carbon atoms of a benzene ring. The phenol family includes the parent compound, phenol, as well as a wide variety of other compounds having additional substituents attached to the phenol ring.

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    • The greater polarity of this bond results in stronger hydrogen bonding and correspondingly higher boiling points, melting points, and water solubility, compared to alcohols.

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    • The pH value of a 0.1 M aqueous solution of phenol is about 5.

    • The higher acidity of phenols compared to alcohols is due to the electron-withdrawing effect of the phenyl ring compared to the R group of an alcohol.

    • The negative charge on the oxygen atom is dispersed around the benzene ring rather than leaving it entirely on the oxygen, as in the alcohol system. This effect is called charge dispersal.

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    • Ethers contain an oxygen atom bonded to two different carbon atoms. The carbon atoms can be either aliphatic or aromatic carbons.

    • The common nomenclature system uses the names of the groups attached to oxygen, in alphabetical order, followed by the word ether.

    • For ethers containing other than simple groups, the IUPAC system is used. In this case the ether is named as a member of some other family, with the more complex group determining the base name. The simpler group and the ether oxygen to which it is attached are treated as an alkoxy (RO) or aryloxy(ArO) group.

    • CH3CH2-O-CH2CH3

    • Diethyl Ether (common) or Ethoxyethane (IUPAC)

    • Common names are used for cyclic ethers:

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    • The boiling point of dimethyl ether is higher than that of the corresponding alkane, propane, because the C-O-C bond is bent and the two carbon-oxygen bonds are polar.

    • This effect falls off rapidly with increasing alkane chain length. Diethyl ether has the same boiling point as its corresponding alkane, pentane.

    • Ethers as a class have much lower boiling points than alcohols due to the lack of hydrogen bonding between molecules.

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    • Intramolecular dehydration to form an alkene competes with intermolecular dehydration to form an ether:

      • Alkene is the major product at 180° C

      • Ether is the major product at 140° C

    • Secondary and tertiary alcohols do not form ethers when heated with an acid catalyst. The attached substituents prevent two molecules from approaching each other closely enough from intermolecular dehydration to occur.

    • This effect is called steric hindrance.

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    • Thiols, or mercaptans, are the sulfur analogues of alcohols. as well as an alkene:

    • The –SH group is called the mercapto, or sulfhydryl, group.

    • The IUPAC nomenclature system adds the ending –thiol to the name of the alkane, but without dropping the final –e.

    • Thiols have distinct odors and flavors, often disagreeable (skunk). The aromas and flavors of garlic and onions are due largely to disulfides.

    • Thiols have considerably lower boiling points than those of alcohols, even though thiols have higher molecular masses, because thiols do not possess the strong hydrogen bonding of alcohols.

    • Thiols are weak acids but much stronger than alcohols, because the S-H bond is weaker than the O-H bond, a consequence of the larger size of sulfur compared with oxygen. However, thiols are weaker acids than phenols.

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    • • Disulfides are easily reduced back to thiols by many reducing agents:

    • In the preceding equations, the abbreviations (O) and (H) indicate

    • general conditions for selective oxidation and reduction, respectively,

    • without specification of the exact oxidizing or reducing agent.

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    • The reactions to form disulfides and insoluble heavy metal salts are of physiological importance.

      • Many proteins contain thiol groups and disulfides critical to maintaining their three dimensional shape and their proper functioning.

      • Lead and mercury salts react with thiol groups, altering the physiological functioning of the proteins involved.

      • A common source of lead is from flakes of paint manufactured prior to 1980. The use of lead tetraethyl as a gasoline additive also introduces lead into the environment, however, its use has been phased out.

      • Mercury from insecticides and other sources enters the food chain through fish from contaminated rivers, lakes, and streams. Mercury has a physiological effect similar to that of lead.